The invention relates to an imaging apparatus such as video camera, digital still camera, or the like using an image pickup device and, more particularly, to an imaging apparatus having a white balance function and its method.
In recent years, video cameras have been spread and a video camera by which an image of higher picture quality can be easily photographed and which can be readily handled is demanded. As one of important requirements for making the image to have the high picture quality, it is necessary to prevent color deviation (hereinafter, referred to as white balance deviation) of an object to be photographed in the image. For example, it is important to avoid the occurrence of the color deviation (what is called coloring) such that a white portion of the image is colored. For this purpose, the video camera is equipped with an automatic white balance function for automatically correcting the color deviation (white balance deviation).
As an automatic white balance function which is used in the video camera or the like, white balance control can be mainly classified into two types: what is called an external photometric type such that control for directly detecting a color temperature of an illumination light source by a color temperature sensor or the like and correcting the white balance deviation on the basis of a detection result (hereinafter, such control is referred to as white balance control) is made; and what is called an internal photometric type such that the white balance deviation of an object to be photographed is detected from a video signal or the like and the white balance control is made on the basis of a detection result. In recent years, the internal photometric type is the mainstream in view of simplicity of a construction, costs, and the like.
As a method for the white balance control of the internal photometric type, there is a general method of correcting the white balance deviation by constructing a feedback loop such that chrominance signals such as color difference signals are formed from a signal (that is, image pickup signal) which is obtained by photographing an object, a white portion is discriminated from the chrominance signals, a white portion range of an image is set, and the chrominance signals included in the white portion range are extracted and integrated, thereby detecting the white balance deviation, and gains of the chrominance signals such as primary color signal R (red), primary color signal B (blue), and the like are controlled on the basis of a result of the detection.
As an example of such a method, for instance, there is a technique disclosed in Japanese Patent No. 3193456. According to white balance control in such a technique, two color difference signals (R−Y) and (B−Y) and a luminance signal Y are formed from primary color signals of R, G (green), and B which are obtained by color-separating an image pickup signal that is outputted from an image pickup device. Further, chrominance signals (R−B) and (R+B−2Y) obtained by base-converting the color difference signals by executing adding/subtracting processes to the color difference signals (R−Y) and (B−Y) are formed. A level of a threshold value provided in accordance with a level of the luminance signal Y is compared with those of the chrominance signals (R−B) and (R+B−2Y), respectively, thereby discriminating an achromatic portion (white portion) of the object. An extracting region (white portion extracting range) of the chrominance signals (R−B) and (R+B−2Y) is set. The chrominance signals (R−B) and (R+B−2Y) in the extracting region are extracted and integrated, respectively. The white balance deviation of the achromatic portion (white portion) of the object is detected from the signals obtained by the integration. By making the white balance control for changing the gains of the primary color signals R and B in accordance with the detected white balance deviation, the white balance deviation is corrected.
According to such white balance control, the chrominance signals included in the specific white portion extracting range on color temperature signal coordinates are regarded as signals according to the white balance deviation of the achromatic portion of the object, and the feedback control is made so as to correct the white balance deviation on the basis of those signals. Therefore, even if a signal of a chromatic portion in which the object is colored is included in the white portion extracting range (there is a case where if a photographing environment change and a color temperature of the light source changes, for example, even in the chromatic portion which is slightly colored, the levels of the chrominance signals (R−B) and (R+B−2Y) are equal to or less than the threshold value), there is a case of occurrence of an erroneous operation such that such a chromatic portion is regarded as an achromatic portion and the correcting operation of the white balance deviation is executed.
To reduce such an erroneous operation, in the white balance control according to the technique disclosed in the Japanese Patent No. 3193456, for example, there is used a method whereby the white portion extracting range is finely set to a plurality of regions every temperature, and by sequentially switching those regions, it is avoided that the signal of the chromatic portion of the object is included in the white portion extracting range.
However, as mentioned above, there is actually a case where it is difficult to accurately separate the white balance deviation of the achromatic portion of the object from the chromatic portion which is slightly colored. For example, in the case where an object is photographed so that a human face (object of a light skin color) occupies the whole picture plane, a portion of the human face is erroneously discriminated as an achromatic (white) portion and the white balance deviation is corrected. Thus, the erroneous operation of the correction of the white balance deviation such that an image in which the human face is faded into blue is obtained occurs.
As mentioned above, although the conventional white balance control is effective to the object having various saturation in the picture plane, there is a case where the proper correction of the white balance deviation cannot be performed to the picture plane in which the object having unbalanced saturation, particularly, the chromatic object in a light color occupies at a large ratio.
As a method of reducing the erroneous operation of the correction of the white balance deviation to the object having the unbalanced saturation, for example, a technique disclosed in Japanese Patent No. 2532968 has been known. In white balance control according to such a technique, an angle of view of a camera lens and a distance to the object are obtained, an effective area of an image pickup picture plane is obtained, and time constants of gain control of chrominance signals are changed in accordance with the effective area. If the effective area is small, a method of limiting or stopping the gain control of the chrominance signals is used. Therefore, in the case of photographing by setting the human face so as to be displayed on almost the whole image pickup picture plane, either an angle of view is narrowed due to the zooming operation or the distance to the object is close in such photographing. Therefore, the effective area in this case becomes small and, at this time, the control of the chrominance signals is limited or stopped. Thus, it is possible to prevent the human face from being faded into light blue.
However, in the conventional technique disclosed in Japanese Patent No. 2532968 mentioned above, in the case of photographing so that the white object or the object of the light source occupies almost the whole picture plane, since the effective area is small, the correcting operation of the white balance deviation is limited or stopped. That is, in spite of the fact that the white object exists, the white balance control is not made. There is, consequently, a problem such that the white balance deviation occurs in the white portion and picture quality of an image deteriorates.
In the case where the distance to the object is far and the object having various saturation exists in the picture plane, color components are not always equivalent in the picture plane. Therefore, if the white balance control is made in such a state, although the slight white balance deviation occurs, since the white portion is small on the picture plane, it is hardly conspicuous. On the other hand, in a photographing state where almost the whole picture plane is occupied by the white object by zooming or approaching the object, unless the white balance control is made, even if the white balance deviation is small, since the object occupies the screen at a large ratio, the white balance deviation is conspicuous.